1. Field of the Invention
The present invention relates to surgically implantable prosthetic replacement devices for joints in the hand and toe. More particularly, the present invention is directed to interphalangeal and metacarpal-phalangeal joints that have been subjected damage due to externally applied forces, such as in an accident, and/or advanced diseases such as rheumatoid arthritis, ankylosis and the like.
2. Description of the Prior Art
The metacarpophalangeal joint is crucial for hand function. However, this joint is frequently affected by debilitating circumstances. The metacarpophalangeal joint is the articulation between the metacarpal and phalange bones in the hand and comprises a metacarpal head, the proximal phalanx, volar plate, two collateral ligaments, two accessory collateral ligaments and the sagittal band. A normal joint has the appearance of a ball-and-socket with three degrees of movement. Primarily motion is in the flexion, and extension direction, but is also capable of minor abduction and adduction rotation.
As can be seen, the metacarpophalangeal joint is complex. In recent years, surgically implantable prosthetic replacement devices have been developed to replace a damaged metacarpophalangeal joint, however, these devices have had varying degrees of success. An extensive medical literature has developed parallel to the patent literature in which numerous devices are described. There are a number of patents that specifically address various structures to replace the joints within the finger, particularly between the phalangeal bones of the hand.
Such devices can comprise single body construction or can be comprised of two or more articulating parts hinged or joined together in a variety of ways. One-piece prosthetic devices generally are composed of a flexible elastomer, such as silicone rubber, are described in U.S. Pat. Nos. 3,462,765; 3,593,342 and 3,681,786. These appear to be suitable as long as the integrity of the device is retained but may be subject to mechanical fatigue over time. Fracture problems have been reported in the medical literature. One problem with this type of prosthesis is that there is little or no lengthwise “play” or “give”. This longitudinal rigidity can result in loosening the attachment of such devices to the bones.
Other devices utilize two or more components. For example, U.S. Pat. No. 4,156,296 discloses an endoprosthetic device for placement between metatarsal and phalangeal bones having a convex, part-spherical bearing surface having a projecting stem for securing the proximal component into the metatarsal. The distal portion of the device has a concave, part-spherical bearing surface with a projecting stem for securing the distal component into the end of the phalanx adjacent to the first metatarsal. The engagement of the two components forms a less-than-hemispherical articulation. Similar to this construction are devices disclosed in U.S. Pat. Nos. 4,231,121; 4,242,759; and 4,642,122 to identify but a few.
U.S. Pat. Nos. 5,133,761; 5,147,386; and 6,352,560 disclose multi-component finger joint prosthesis having a first part with a cylindrical socket and a second part with a cylindrical head. The cylindrical head and socket are of complementary form, in such a manner that together they are able to form a linear hinge. In the '761 and '560 patents, the cylindrical head is provided with a radial thickened section that extends in the circumferential direction. The cylindrical socket is provided with a radial recess extending circumferentially and which engages the radial thickened section in bearing contact inside the cylindrical socket. The radial thickened section and radial recess are arranged centrally and are formed so as to be self-aligning with respect to one another.
These synthetic joints do not always replicate the characteristics of a human phalangeal joint. As noted above, the human phalangeal joint has the ability to flex in one plane just as one may curl the finger. The human joint can endure lateral movement and slight twisting. Additionally, a form of longitudinal play is possible along the length of the phalangeal joint. These four degrees of motion have been difficult to achieve in a synthetic joint which is also durable, easily assembled, modular in approach so that an optimal bone-to-implant interface can be achieved, easily put into the human body, and which best replicates the joint that it replaces.
There is a great need in the prosthetic industry for phalangeal replacement joints, as well as other joints, which can achieve all of the desirable attributes articulated above. It is an object of this invention to provide a two part joint prosthesis having substantial freedom from mechanical fatigue and permitting small longitudinal hyperextension without loss of function.